The present invention relates to an improved vapor-compression heat pump system and method and, more particularly, to a vapor-compression heat pump with a two-phase/two-component solution which is compressed by a novel liquid-vapor compressor to provide a lightweight, efficient and reliable thermal control system.
A conventional heat pump system uses the basic vapor compression cycle schematically shown in FIG. 1 in which a refrigerant evaporates at low pressure to provide the desired cooling. The refrigerant vapor is then slightly superheated in the evaporator to avoid knocking associated with liquid compression. Thereafter, the superheated refrigerant vapor is compressed to a higher pressure to raise the condensation temperature so that heat can be transferred to another environment as the compressed superheated refrigerant vapor condenses into a subcooled liquid. This liquid is throttled in an expansion device to a two-phase mixture which then enters the evaporator to complete the process. FIG. 1A shows a modification of the basic vapor compression cycle with a heat recovery heat exchanger to improve the performance.
Also known is a heat pump system using a similar cycle commonly referred to as a vapor-compression cycle with solution circuit. FIG. 2 schematically shows this cycle, and FIG. 2A further shows that cycle but with a heat recovery heat exchanger to improve the performance in a manner similar to FIG. 1A.
For purposes of the following description of the solution circuit cycle, the "absorbent" is defined as a liquid capable of absorbing a refrigerant in vapor state to form a liquid solution. "Solution" is defined as the absorbent with a certain percentage of refrigerant in solution. The "mixture" is defined, unless otherwise indicated, as a two-phase, two-component liquid-vapor mixture consisting of liquid absorbent and refrigerant solution mixed with superheated refrigerant vapor. "Solution concentration" refers to the relative amounts of absorbent and refrigerant, and is based on the absorbent concentration. A weak solution contains more refrigerant in solution than does a strong solution. Pure absorbent would be the strongest possible solution, whereas pure refrigerant would be the weakest possible solution.
In this vapor-compression heat pump with solution circuit cycle, a generator is used in lieu of the pure-refrigerant, or single-component refrigerant, evaporator, and a single-phase, two-component weaker solution (e.g. an all liquid solution with absorbed refrigerant) enters the generator with resulting generation and dispersion of some or all of refrigerant vapor from the solution mixture to provide the cooling of, for example, a space. Because refrigerant is driven off the solution, the concentration of absorbent is increased which results in a stronger solution. The liquid and vapor are physically separated in this cycle, and both the heat of vaporization of the refrigerant and the heat of dissolution (the reverse of the heat of solution) are absorbed in the generator which is a combination of a heat exchanger and a liquid separator. The superheated vapor which is driven off and separated is compressed to higher pressure, and the remaining stronger liquid solution (i.e., the absorbent-and-refrigerant liquid solution) is compressed using a liquid pump.
The separate liquid and vapor streams at the higher pressure in the conventional solution circuit cycle are then combined in the absorber, where the absorption of the refrigerant vapor into the solution, causes heat to be transferred to the environment via heat of condensation plus heat of solution. The absorber is thus both a heat exchanger for transferring, or rejecting, heat to the environment as well as a device for exposing the concentrated liquid solution to the vapor. One type of known absorber utilizes a falling film of liquid solution which passes through the vapor as the liquid solution falls. The mixture accumulates at the bottom of the falling film where a heat-transfer coil or pipe transfers the heat out of the solution and into a pumped coolant flowing through this cooling coil.
After leaving the absorber shown in the embodiment of FIG. 2, or in the alternative embodiment of FIG. 2A using the heat recovery heat exchanger, the pressure of the resulting liquid two-component mixture is then decreased, via a throttling valve, and the mixture reenters the generator to complete the cycle. The vapor-compression cycle with solution circuit has an improved COP.sub.c (Coefficient of Performance) because the latent heat has increased, thereby increasing the cooling capacity, for very little additional work. The system is not very practical, however, because it requires constant balance of the liquid solution flow and compressed vapor flows which combine in the absorber. Unfortunately, the flow rates of the vapor and the liquid solution change with cooling temperature and load, and the liquid pump and vapor compressor typically have different flow-versus-pressure characteristics. The end result is that even under typical variable loads, the vapor compressor and liquid pump discharge pressures do not always match causing significantly reduced performance or even temporary failure of the system. Complex pump, compressor, and/or by-pass control logic have not effectively resolved this very volatile control problem, and these control methods reduce performance and significantly increase complexity.
Another significant shortcoming of this vapor-compression cycle with solution circuit is that the absorber must expose the vapor to the liquid solution, via a falling film or some other mechanical apparatus, adequately to mix the vapor. Because of the finite time required for the chemical or physical absorption to occur, there must be sufficient residence time to allow the refrigerant to absorb into the bulk liquid mixture. This residence time requires a significant amount of space which means that absorbers are disadvantageously quite large, a serious disadvantage in applications where space and weight are critical. Similarly, generators must provide sufficient time and free surface to allow the desorbed vapor to physically migrate out of the solution; again, this requires a significant amount of space. Generators, which are not quite as large as absorbers, are the second largest component in the system. Large size also usually means heavier and more expensive, disadvantages which are severe where space, weight and cost are important considerations.
It is an object of the present invention to overcome the disadvantages of the known vapor-compression heat pump with solution circuit while retaining the advantages of that system. This object has been achieved by a system which compresses the two-phase, two-component solution together without separating them. That is, the liquid refrigerant/absorbent solution and the superheated vapor are compressed together as a mixture rather than using a separate compressor and pump. As a result, it is not required that the low temperature generator-like heat exchanger also be a separator, and the high-pressure, heat rejection absorber-like heat exchanger does not need to distribute the vapor to the solution, since they always remain in contact, i.e. always remain well mixed. In fact, both of these heat exchangers can be shell-and-tube-heat exchangers or any other type of compact heat exchanger in lieu of a larger falling-film type absorber or generator. Furthermore, the disadvantage of a finite absorption/desorption time which leads to large absorbers and generators in adsorption heat pumps and vapor-compression heat pumps with solution circuits is an advantage in the chemical/mechanical heat pump of the present invention.
The major basic components of the chemical/mechanical heat pump system of the present invention comprise an absorbent/refrigerant working fluid mixture of two miscible fluids which, when effected by heat addition, form a pure vapor fluid and a remaining liquid fluid; a low temperature heat exchanger which allows for the desorption and vaporization or chemical reaction of refrigerant from the liquid mixture to form a liquid-vapor mixture; a two-phase compressor which compresses this liquid-vapor mixture; a high temperature heat exchanger which allows the compressed mixture to reject heat as the vapor is recombined with the liquid solution; and a throttling valve which drops the liquid solution pressure (ideally constant enthalpy), so that the working fluid can once again be desorbed and vaporized or chemically reacted in the low temperature heat exchanger.
The working fluid for the cycle of the present invention is an absorbent/refrigerant mixture. For the purposes of the following description of the cycle of the present invention, the absorbent can be a liquid capable of absorbing refrigerant vapor to form a liquid solution, a solid particle absorbent or adsorbent suspended in a liquid carrier capable of absorbing or adsorbing refrigerant vapor, or a liquid compound capable of reversible chemical reaction with refrigerant vapor to form a new liquid-phase compound. In connection with the foregoing, absorption refers to the penetration of one substance into the inner structure of another, adsorption refers to the adherence of molecules to the surface of another substance, and chemical reaction refers to the chemical change of the molecule.
The refrigerant of the absorbent/refrigerant mixture can be a compound in the vapor phase. The absorption, adsorption, or reaction of refrigerant vapor into solution rejects a heat of mixing (heat of absorption, heat of adsorption, heat of solution, or heat of reaction) and heat of vaporization during the mixing process. Reversing the process, namely the liberation of the refrigerant from the liquid solution, requires the addition of the heat of mixing and the heat of vaporation to cause this vaporization of the refrigerant vapor from the liquid solution (i.e., the refrigerant is driven from the solution as a superheated vapor).
The working fluid mixture has greater latent heat capability, when compared to a pure or single-component working fluid, because a pure fluid (or refrigerant) has only latent heat of vaporization, whereas the absorbent-refrigerant mixture of the present invention utilizes the heat of vaporization and the heat of mixing (i.e. heat of absorption, heat of adsorption, heat of solution, or heat of reaction), which can be much greater than the latent heat of vaporization alone.
According to the present invention, a substantially liquid solution (i.e. liquid absorbent, solid absorbent/adsorbent in a liquid carrier or a liquid compound with the capability of reversible chemical reaction) with absorbed/adsorbed or chemically-reacted refrigerant, leaves the high temperature heat exchanger after rejecting heat to the surroundings, and enters the throttling or expansion valve where the pressure is decreased. The solution then enters the low temperature heat exchanger where the heat is transferred into the solution (which provides the desired cooling) and drives some of the refrigerant out of solution. This solution is typically now a superheated refrigerant vapor and liquid solution mixture which is not separated, but instead is kept together and compressed to a desired higher pressure. As the mixture of absorbent and refrigerant is compressed, the refrigerant vapor will begin to recombine with the absorbent in the compressor. This process is, however, substantially adiabatic (insulated) because the small residence time and heat transfer area of the compressor limits the heat transfer. Consequently, the discharge temperature of the compressor is increased more than that of a conventional vapor-only compression system. When the compressed absorbent/refrigerant mixture reaches the high temperature heat exchanger, the heat is rejected to the surroundings. As the solution cools, additional vapor is condensed and recombines (i.e. absorbs, adsorbs or reacts) with the liquid solution. The fluid thereafter leaves the high temperature heat exchanger as a liquid absorbent solution which is then throttled before returning to the low temperature heat exchanger.
There are rather significant advantages of the chemical/mechanical heat pump of the present invention over the conventional vapor compression cycle with solution circuit. In the typical vapor compression cycle with solution circuit, the heat adsorbed in the generator at low temperature causes the vaporization and desorption of the refrigerant from the liquid solution and the two fluids are then physically separated. The refrigerant vapor is compressed and the liquid solution is pumped to higher pressures where they are combined. Unfortunately while this cycle has the potential for higher performance, the complexity of the separation process combined with the complexity with matching the compressor and pump discharge pressures under continually varying flow ratios makes this system of limited practical application. In the present invention, a compact heat exchanger, instead of an absorber, can be used to transfer the energy necessary to liberate the vapor from the solution; however the liquid solution and superheated vapor (which has been driven from the solution by the addition of heat) are not separated. At the conditions in this heat exchanger, there is an equilibrium concentration of the superheated vapor with the solution. As the mixture of liquid solution and entrained superheated vapor is compressed, however, the vapor will no longer be in equilibrium concentration with the solution and instead will begin to recombine with the absorbent solution which results in the rejection of energy, from the heat of mixing and heat of vaporization.
The absorption of the vapor back into the liquid in the system of the present invention begins to occur as soon as the mixture is compressed and is quite different from the vapor compression cycle with solution circuit where the solution and vapor streams have been physically separated, so they can be pressurized (vapor compressed and liquid pumped) and recombined in the absorber. In the conventional vapor-compression cycle with solution circuit, the absorber must physically mix these components and provide the necessary residence time for this recombination to take place so that heat energy of mixing and vaporization can be rejected in the absorber's heat exchanger. In the chemical/mechanical cycle of the present invention, however, the solution and vapor are always well mixed and the recombination begins as soon as the fluids begin to be compressed.
We have found it to be essential that the absorbent/refrigerant mixture must be compressed and rejected from the compressor before significant absorption can take place. Ideally, the compression is instantaneous so that no vapor has time to get back into solution. Conversely, if the compression were very slow, all the vapor would have a chance to get back into solution, and the performance would be severely reduced. The compression can be accomplished quite rapidly, resulting in essentially very little recombination of vapor and solution in the compressor.
A significant benefit of our approach is that a liquid vapor separator is not used, a large adsorber (large because of the need to get the vapor in contact with the solution) is not used, and a larger generator (larger because of the need to allow the solution time to desorb the vapor from solution) is not used. In other words, in an unfortunate thermodynamic characteristic, namely the finite time necessary to absorb or desorb the vapor into or out of the solution, had traditionally resulted in large absorbers and generators. However, in the chemical/mechanical heat pump configuration, this same recombination time requirement is eliminated, resulting in a smaller overall system, with far fewer components.
In order to achieve the foregoing advantages with the cycle of the present invention, the liquid-vapor mixture must be compressed without knocking. Conventional compressors compress only a vapor, however, and, in fact, the inlet vapor to a compressor is usually superheated to avoid any knocking caused by the attempt to compress a liquid. We have been able to achieve in a liquid absorbent and vapor refrigerant system two phase compression without knocking by the use of a sliding vane compressor of the type generally described in U.S. Pat. No. 5,310,326; the disclosure of which is incorporated herein by reference.
Alternatively, a solid particle absorbent or adsorbent in a liquid carrier (such as metal hydride absorbent and hydrogen refrigerant) can be used in which the solid absorbent is only 20% of the liquid mixture and only 5% of the hydrogen refrigerant is adsorbed on the absorbent (therefore 1.0% of the solution is vapor by mass, but 13% of the solution is liquid by volume). Although a two-phase compressor is still required, the effective latent heat is 63.0 BTU per pound of hydride-carrier-hydrogen mixture (which is comparable to the latent heat of refrigerant-22 at 87 BTU/lb. or refrigerant-12 at 64 BTU/lb.) but the pressure ratio is only 1.7 for the hydride slurry as compared to 4.3 for refrigerant-12 or 4.2 for refrigerant-22. The lower pressure ratio results in reduced compressor work for the same mass flow rate. Therefore, the metal hydride absorbent example demonstrates significantly improved performance because the lower pressure ratio results in reduced work for essentially the same cooling capacity per unit mass flow of working fluid.
Since the coefficient of cooling performance, COP.sub.c, is defined as the cooling capacity divided by the work, the substantial increase due to the heat of mixing of the working fluid in the present invention increases the cooling capacity significantly. Because liquids are relatively incompressible, the additional work to compress the liquid solution over the larger pressure ratio is reduced. Alternatively, for the embodiment using solid absorbent/adsorbent materials in a liquid carrier (such as metal hydrides), the latent heat is not significantly improved compared with pure refrigerant, but the compressor work is significantly reduced because of the significantly lower pressure ratio. Therefore, for both types of working fluid mixtures, the COP.sub.c increases significantly compared with a conventional single fluid vapor-compression system.
We have also discovered several characteristics that make for an ideal absorbent/refrigerant mixture (or pair) to effect the objectives and advantages of the present invention. For instance, the absorbent liquid must be relatively non-volatile at the generator temperature and have a high affinity for the refrigerant vapor. Both the absorbent and refrigerant must be condensable in the operating temperature range and must not have an excessive vapor pressure. Both fluids should be inert at system temperatures. The fluids should have a high density and a high heat of solution. The fluid must also be compatible with all material it will encounter in the heat pump system and be environmentally safe and non-ozone depleting.
The innovative heat pump system according to the present invention is a substantially improved vapor-compression refrigeration/heat pump using the heat of mixing (i.e., heat of absorption/adsorption, heat of solution or heat of reaction) to enhance the thermal performance, thereby reducing space requirements, component weight and cost. In addition to the latent heat of vaporization alone, the heat of mixing, is advantageously used to increase the cooling capacity and performance.
A component of the improved heat pump system of the present invention is a two-phase, positive displacement compressor for compressing the mixture of the liquid absorbent-refrigerant solution and vapor to a high pressure. The compressor has a composite bore composed of cycloidal and circular curves based upon a recognition of fluid compression and vane motion dynamics, which is compact and lightweight, for two-phase flow and determined based upon calculations of radial components of vane velocity and acceleration, and sealing required between the rotor and bore. With appropriate transitions between the circular and cycloidal curves, dynamic forces are reduced to a minimum. A rotor is located eccentrically inside the bore and has four symmetric vane slots connected each at the bottom. Four self-lubricating vanes slide in the slots following the bore contour as the rotor rotates in a manner generally described in U.S. Pat. No. 5,310,326, resulting in variable volume chambers needed for drawing-in and compressing the two-phase/two-component mixture. A circumferential inlet streamlines the inlet flow and reduces the fluid impact on the bore. A circular sealing area on the bore advantageously helps to seal the fluids effectively, and the cycloidal curves provide vanes with a smooth movement. Compression has been successfully carried out for a two phase mixture with a 30% (by weight) liquid solution at a pressure ratio of approximately 4. Compression of higher weight percent liquid concentrations is possible at lower pressure ratios as is the case, for example, with metal hydrides in a liquid carrier.
To achieve the chemical/mechanical heat pump, the liquid solution, mixed with refrigerant vapor, is driven to the high pressure side by the two-phase compressor of the present invention. Prior to the present invention, compressors could not successfully, i.e. without damage, compress a two phase mixture to high pressure. In fact, conventional compressors were typically safeguarded from two phase compression because it reduced compressor life due to fluid impact and/or compressor knocking. For two-phase flow system in thermal, chemical, and petroleum industries, the traditional way to handle two phase compression was to separate the fluid from the mixture, then use an additional pump to drive the liquid while a compressor pressurized gas or vapor. This former method often led to difficulties in process control such as matching the dynamic characteristics of the pump and the compressor, or accommodating different flow ratios of pump and compressor. In addition, the known compressors did not address the problem of knocking. Positive displacement compressors were more susceptible to fluid knocking problems which caused damage from enormous pressure during operation. The compressor configuration of the present invention effectively avoids these problems.